This application claims priority to German application 100 60 718.7, filed Dec. 7, 2000, herein incorporated by reference.
The invention relates to a process for preparing nondusting free-flowing pigment concentrates with removal of the water from pigment suspensions comprising polymers and dispersants in a fluidized bed drier. The resulting bead-form pigment concentrates are nondusting and readily free-flowing. The invention further embraces the use of pigment concentrates prepared in this way for coloring plastics and polymers.
Related Applications
Pigment concentrates and processes for preparing them for the abovementioned applications can be found in the prior art.
For instance, DE-B-12 39 093 describes carrier materials based on a mixture of an amorphous ethylene-propylene block copolymer and a crystalline polypropylene for preparing pigment concentrates. DE-A-26 52 628 relates to the use of polypropylene waxes having a viscosity of from 500 to 5 000 mPas (170xc2x0 C.) and an isotactic fraction of from 40 to 90%.
Plastics are normally colored using pigment concentrates (pigment masterbatches). The pigment concentrates, prepared by extrusion processes, contain the corresponding pigment in concentrations of from about 20 to 70% by weight, a polymeric carrier, and usually different kinds of substances (waxes/wetting agents/dispersants) which assist the process of incorporating the pigments and ensure highly uniform distribution (dispersion) of the pigments.
In certain cases, the dispersing of organic pigments is circumvented through the use of pigment flush pastes. The preparation of what are known as flush pastes is prior art and is described in more detail by W. Herbst and K. Hunger in xe2x80x9cIndustrielle Organische Pigmentexe2x80x9d, 2nd edition, pp. 91-92, VCH Verlagsgesellschaft mbH, D-6945 Weinheim, 1995. In this technology, the aqueous pigment presscake normally obtained initially in pigment preparation is not dried and ground but is instead treated in a xe2x80x9cflushxe2x80x9d process with oily binders, such as alkyd resins, mineral oils, cellulose acetobutyrate or other suitable substances. The water on the pigment surface is displaced by the organic substances. By flushing, therefore, the process steps of pigment preparation that lead in particular to the formation of agglomerates, namely drying and grinding, are avoided.
Flush pastes are prepared in an internal compounder. Presscake and dispersing waxes are mixed in the compounder; the water is removed by heating and reduced pressure; the wax goes onto the pigments. This is a batch process, and the pigment preparation must subsequently be standardized.
In the film segment, bittiness disrupts the visual appearance and leads to cracking and to unwanted light scattering effects.
In the production of polymeric fiber, pigment agglomerates cause clogging of the extruder""s melt grids and cause fiber breakage, which is generally accompanied by prolonged downtimes and increased plant cleaning demand.
In DE-A-195 16 387, highly effective dispersing is achieved by means of a dispersant comprising a mixture of different polyolefin components and special polyacrylic esters.
Pigment preparations in the solid state may be prepared by mixing as a dry blend and/or by melt mixing in appropriate extruders or compounders.
Normally, the pulverulent organic or inorganic pigments are mixed together with granular or pulverulent polymers and corresponding waxes. The waxes too are used in powder form. They are obtained either by spraying techniques or milling processes. The particle sizes of these waxes are generally  less than 1 000 xcexcm. The particle size distributions vary, of course, depending on the process used.
For optimum dispersing, up to 40% by weight of wax is needed, depending on pigment type. Since excessive amounts of wax lead again and again to instances of migration and deposits on the dies, there is particular interest in preparing pigment concentrates comprising effectively dispersed pigments with very small fractions of waxes.
The invention provides in a first embodiment a process for preparing nondusting, free-flowing, organic pigment concentrates from pigment presscakes obtained in pigment preparation, with the addition of wetting agents and dispersants, the water being removed in a fluidized bed process.